1. Field of the Invention
The present invention relates to a method of through-etching a substrate, which is an important technique in manufacturing a three-dimensional microelectromechanical system (MEMS).
2. Description on the Related Art
A microelectromechanical system (MEMS) is referred to as an ultra-small system or a system made of ultra-small machine parts of several micrometers or several millimeters. Also, the MEMS is an integrated microelement that is coupled with electric machine parts and mechanical machine parts and manufactured via batch-type process techniques used in manufacturing semiconductor integrated circuits.
Today, use of the MEMS has been rapidly spreading because it is a compact, portable, and efficient thermodynamic energy system. In particular, a lot of interest is centered on developing the structure, functions, and techniques of manufacturing a three-dimensional (3-D) MEMS that is a multi-layered structure of several silicon substrates, each having a recess region and a penetration region of a regular form. For example, techniques of through-etching a silicon substrate have been continuously developed.
A lot of effort to develop the technique of through-etching a substrate has been undertaken by the Gas Turbine Laboratory of Massachusetts Institute of Technology (MIT) since 1995. For example, the recent trends regarding techniques of through-etching a substrate are introduced by Ravi Khanna et al. of MIT in a report entitled xe2x80x9cMicrofabrication Protocols for Deep Reactive Ion Etching and Wafer-Level Bondingxe2x80x9d [Sensors, April 2001]. Also, an example of a method of through-etching a substrate applied to a power MEMS is disclosed in a report entitled xe2x80x9cDemonstration of a Microfabricated High-Speed Turbine Supported on Gas Bearingsxe2x80x9d written by Luc. G. Frechette et al. of MIT [Solid-State Sensor and Actuator Workshop, June 2000]. Further, an example of a method of through-etching a substrate applied to micro gas turbine is disclosed in a report entitled xe2x80x9cA Six-Wafer Combustion System for a Silicon Micro Gas Turbine Enginexe2x80x9d [Amit Mehra et al., Journal of Microelectromechanical Systems, vol. 9, No. 4, December 2000].
FIGS. 1 through 5 are cross-sectional diagrams explaining a conventional method of through-etching a substrate used in the MEMS, which is disclosed in xe2x80x9cMicrofabrication Protocols for Deep Reactive Ion Etching and Wafer-Level Bondingxe2x80x9d introduced by Ravi Khanna et al. of MIT [Sensors, April 2001].
Referring to FIG. 1, a photoresist layer 12 is formed on a wafer substrate 10 to be etched. Next, a photoresist pattern, which exposes a portion of the wafer substrate 10, is formed by performing an exposure process and a developing process, which are generally adopted during a process of manufacturing semiconductor devices, on the wafer substrate 10 having the photoresist layer 12.
Referring to FIG. 2, with the photoresist pattern as an etching mask, a portion of the wafer substrate 10 is dry etched to form a recess region 10a of a trench shape in the wafer substrate 10. Then, the photoresist pattern 12 is stripped to be removed.
Referring to FIG. 3, in order to penetrate the recess region 10a, the side of the wafer substrate 10 having the recess region 10a is adhered to a handling wafer 20 via a photoresist layer 14. Preferably, the handling wafer 20 is formed of a hard material such as quartz or silicon.
The reason why the handling wafer 20 is used during the through-etching of the wafer substrate 10 is to prevent helium gas from leaking into an etching chamber. In detail, the wafer substrate 10 to be etched is loaded onto a stage installed at a central lower portion of the etching chamber and then is dry etched using plasma. At this time, in order to cool the heat generated in the wafer substrate 10, helium gas, which is cooled to a predetermined temperature, is introduced into the stage to reach the bottom of the wafer substrate 10. However, when a hole is formed at a portion of the wafer substrate 10 that is passed through, the introduced helium gas leaks into the etching chamber, thereby polluting the etching chamber and changing the etching process conditions. Therefore, the handling wafer 20 is used to prevent the leakage of the helium gas into the etching chamber.
After adhering the wafer substrate 10 to the handling wafer 20, a photoresist layer 16, as shown in FIG. 4, is formed on the side opposite to the wafer substrate 10 having the recess region 10a. Then, as described above, the exposure and developing processes are performed on the photoresist layer 16 to form a photoresist pattern exposing a portion that includes the recess region 10a of the wafer substrate 10. Next, a combination of the wafer substrate 10 having the photoresist pattern and the handling wafer 20, is loaded onto the stage of the etching chamber. Then, with the photoresist pattern as an etching mask, the wafer substrate 10 is dry etched using plasma so as to be through-etched.
Then, referring to FIG. 5, after through-etching, the combination of the wafer substrate 10 and the handling wafer 20 is unloaded from the etching chamber, the handling wafer 20 is detached from the wafer substrate 10, and then the photoresist layers 14 and 16 are stripped to be removed.
The conventional method of through-etching a wafer substrate using a handling wafer, however, has the following problems:
i) resist burning may occur. That is, bubbles may form in the photoresist layer 14 positioned between the wafer substrate 10 and the handling wafer 20 during a plasma process;
ii) wafer breakage occurs. As shown in the xe2x80x9cAxe2x80x9d portions of FIG. 4, plasma ions hit the surface of the handling wafer 20, rebound therefrom at the end of the process of through-etching the wafer substrate 10, and then hit the sidewalls of the wafer substrate 10 again. As a result, the sidewalls of holes formed in the wafer substrate 10 are damaged;
iii) structure erosion occurs. The handling wafer 20 made of silicon or quartz has such a low heat conductivity that the etched wafer substrate 10 cannot be sufficiently cooled by helium gas. Thus a specific portion of the etched wafer substrate 10 is etched rapidly as if it would corrode; and
iv) a lot of time is spent on detaching the handling wafer 20 from the wafer substrate 10. Acetone is used to detach the handling wafer 20 from the wafer substrate 10. However, since the handling wafer 20 was fast-coupled with the wafer substrate 10, several hours are lost in separating the handling wafer 20 from the wafer substrate 10.
To solve the above problems, it is a first object of the present invention to provide a method of through-etching a substrate that is simplified and only uses general techniques that are used in manufacturing semiconductor devices, without adhering a handling wafer to a wafer substrate.
It is a second object of the present invention to provide a method of through-etching a substrate, adopting a material of high heat conductivity rather than a handling wafer of low heat conductivity, by which cooling gas is prevented from leaking during through-etching, thereby sufficiently cooling the etched wafer substrate.
It is a third object of the present invention to provide a method of through-etching a substrate, adopting a material of high electric conductivity rather than a handling wafer of low electric conductivity, by which the flow of ions can be kept to be regular during a plasma dry etching process, thereby preventing the breakage of the sidewalls of an etched wafer substrate.
It is a fourth object of the present invention to provide a method of through-etching a substrate, by which after through-etching the substrate, all unnecessary layers except for the substrate are rapidly removed to shorten the processing time.
To achieve an aspect of the above objects, there is provided a method of through-etching a substrate, including forming a buffer layer on a first plane of the substrate; forming a metal layer on the buffer layer; forming an etching mask pattern on a second plane opposite to the first plane; and through-etching the substrate with the etching mask pattern as an etching mask.
This method may further include forming a recess region on the first plane before forming the buffer layer on the first plane. Also, forming the recess region may be performed by a photolithography which is well-known in manufacturing semiconductor devices.
After through-etching the substrate, this method may further include removing the etching mask pattern, removing the metal layer, and removing the buffer layer.
Preferably, the substrate is formed of a single-crystal silicon, the buffer layer is formed of silicon dioxide, and the metal layer is formed of aluminum.
To achieve another aspect of the above objects, there is provided a method of through-etching a substrate, including forming a recess region of a predetermined depth on a first plane of the substrate; forming a first buffer layer on the first plane of the substrate having the recess region; forming a first metal layer on the first buffer layer; forming a first etching mask pattern on a second plane of the substrate opposite to the first plane, for exposing at least a portion of a region corresponding to the recess region; and through-etching the substrate with the first etching mask pattern as an etching mask.
Preferably, forming the recess region on the first plane of the substrate includes forming a second etching mask pattern on the first plane of the substrate; etching a portion of the substrate with the second etching mask as an etching mask; and removing the second etching mask pattern. Preferably, the second etching mask pattern is a photoresist pattern, or a stacked structure including a second buffer layer and a second metal layer.
Preferably, the first etching mask pattern is a photoresist pattern, or a stacked structure including a third buffer layer and a third metal layer.
This method may further include wet-etching the first buffer layer that is exposed by the through-etching of the substrate.
Also, this method may further include, after through-etching the substrate, removing the first etching mask pattern that is the photoresist pattern; removing the first metal layer; and removing the first buffer layer. After through-etching the substrate, this method may further includes removing the third metal layer of the first etching mask pattern and the first metal layer; and removing the third buffer layer of the first etching mask pattern and the first buffer layer.
Preferably, through-etching the substrate is performed by DIRE, and a portion of the substrate, which is penetrated by the through-etching of the substrate, includes the recess region.
According to the present invention, instead of a handling wafer, a metal layer having superior heat conductivity and electrical conductivity is used to sufficiently cool the substrate during the through-etching of a substrate. Also, the flow of ions can be kept to be regular so that a plasma etching process can be successfully performed and pressure applied by a cooling gas can be sufficiently mitigated. Further, with the use of general techniques used in manufacturing semiconductor devices, a process of through-etching the substrate can be simplified and processing time can be shortened.